This invention relates to a UHF electrosurgery system, an electrosurgical instrument and a method of operating an electrosurgery system at UHF frequencies.
It is known to use a needle or narrow rod electrode for cutting tissue in monopolar electrosurgery at frequencies in the range of 300 kHz to 3 MHz. An electrosurgical signal in this frequency range is applied to the electrode, and the electrical current path is completed by conduction through tissue to an earthing plate secured to the patient""s body elsewhere. The voltage applied to the electrode must be sufficiently high to cause arcing and consequent thermal rupture so that tissue adjacent the needle is ablated or vaporised. The arcing heats the active electrode surface to the extent that thermionic effects occur, causing electrical nerve stimulation.
According to the present invention, there is provided an electrosurgery system for the electrosurgical cutting of tissue, the system comprising an electrosurgical generator, a feed structure and an electrode assembly, wherein electrode assembly comprising at least an exposed active electrode coupled to the generator via the feed structure, wherein the generator and the feed structure are capable of delivering radio frequency (r.f) power to the active electrode at a UHF operating frequency, and wherein the active electrode comprises an elongate electrically conductive member which is less than xcex/8 in length, where xcex is the wavelength in air of the delivered power at the said operating frequency. In a preferred system in accordance with the invention, a return electrode is coupled to the feed structure and located adjacent the active electrode so as to be capacitively coupled to tissue adjacent the active electrode when the active electrode is in contact with the tissue. The active electrode preferably has a proximal end attached to the feed structure and a free distal end, with the return electrode also forming part of the electrode assembly and comprising a conductive lamina located laterally of the active electrode and set back from the tip of the active electrode such that when the active electrode is inserted in tissue, the return electrode is adjacent the tissue surface. In the preferred embodiment, the return electrode is covered with an insulating layer to prevent direct electrical contact. Providing the feed structure as the coaxial combination of inner and outer supply conductors respectively coupled to the active and return electrodes produces an arrangement in the form of a coaxial transmission line with an open-circuit end. When the active electrode is applied to the tissue, a UHF field is generated within the tissue which acts as a lossy dielectric, and the tissue surface is capacitively coupled to the return electrode, thereby completing a UHF current path.
Capacitive coupling between the tissue and the return electrode can be enhanced by providing a return electrode which includes a resonant assembly. This provides voltage multiplication at the return electrode, and the consequently increased voltage helps to overcome the high impedance capacitive coupling path between the tissue and the return electrode.
Accordingly, a further independent aspect of the present invention provides an electrosurgical system comprising a generator for supplying electrosurgical power at an operating frequency, a feed structure, and an electrosurgical instrument comprising at least a pair of electrodes, each of which is connected to a conductor of the feed structure, wherein one of the electrodes of the electrosurgical instrument includes a resonant assembly which is resonant at the operating frequency of the generator.
A further independent aspect of the present invention relates to an electrosurgical instrument which may (although need not necessarily) be used as part of with the above system, such an instrument comprising: an active electrode; an outer electrical conductor extending coaxially around the active electrode; an isolating choke connected to the outer electrical conductor, the isolating choke being adapted to prevent the passage of UHF power of a given frequency along the outer surface of the outer conductor; wherein a return electrode is provided by a resonant assembly connected to the outer conductor and located distally of the isolating choke, the resonant assembly having a resonant frequency which corresponds substantially to the frequency at which the isolating choke is operational.
Typically, the system is capable of delivering at least 13 W of UHF power, and a power density at the active electrode of at least 5 W/mm2, this density figure being obtained by dividing the delivered power by the exposed surface area of the active electrode.
In contrast to the delivery of power at frequencies in the range of 300 kHz to 3 MHz which is characterised by arcing causing instantaneous boiling within the cell and very high voltage gradients, at UHF the dielectric behaviour of the tissue becomes significant. Indeed, the dielectric behaviour of the tissue predominates. For example, the real impedance of blood is in the region of 200 ohm cm, whereas the dielectric impedance is in the region of 36 ohm cm at 1 GHz and 14.6 cm at 2.45 GHz. (The relative dielectric constant of blood is about 50.) Accordingly, the resistive loading at 2.45 GHz is less, by a factor of at least 15, than the load presented at conventional electrosurgical frequencies. Since power dissipation is proportional to the square of the load voltage, divided by the equivalent resistance (equivalent to the dielectric loss factor), the applied voltage at 2.45 GHz need be only a quarter of that at low frequencies for the same dissipated power. In other words, a voltage source at 2.45 GHz will deliver 15 times the power of the equivalent low frequency voltage source. The power density is correspondingly 15 times greater.
As a result, for a given level of power dissipation, the voltage can be reduced to an extent such that the voltage gradient in the region of the active electrode is insufficient to promote arcing, and cell rupture occurs due to dielectric heating substantially without attendant nervous stimulation. This property is particularly advantageous in procedures where nerve stimulation can be harmful, such as in neuro-surgery, and surgery on the spine. Cutting or ablation may also be performed without arcing in relatively high impedance tissue, such as fatty tissue.
In this context, it should be noted that at conventional electro-surgery frequencies arcing occurs due to the very high voltage gradients required to achieve cell disruption, and the arcs themselves perpetuate the high voltage gradient due to the desiccation of tissue and consequent raising of load impedance. Tissue within the breakdown range is vaporised by the arcs, causing a gap between the electrode and tissue. The voltage required depends on the gaseous environment, but in air a peak voltage of 300 volts or greater is typical. Cellular disruption occurs due to conduction through the tissue. The much lower voltage gradient produced using the above-described UHF system is due to, additionally, dielectric loss in the tissue surrounding the active electrode. Energy is dielectrically coupled, even across air gaps, i.e. between the active electrode and the surrounding tissue, and between the tissue and the set-back return electrode. Significant power dissipation can be achieved without exceeding the breakdown voltage gradient for air of 3 kV/mm.
A particular advantage of using frequencies in the region of 2.45 GHz is that the dielectric loss is a maximum due to the molecular resonance of water molecules at about this frequency.
To cut tissue, the active electrode penetrates the tissue surface to a depth of, typically, a few millimeters. Since the dielectric constant of tissue is considerably higher than that of air, the effective electrical length of the electrode decreases when in tissue, to the extent that the electrode may act as a step-up transformer, producing a standing wave voltage peak in the region of its tip. One of the advantages of arranging for the active electrode length to be less than xe2x85x9 of the operating wavelength in air is that when the electrode is withdrawn, such standing wave maxima are no longer present. Accordingly, the voltage at the electrode reduces upon withdrawal, in contrast to conventional, low frequency, electrodes in which the comparatively high generator source impedance results in the output voltage increasing due to the increased load resistance in air as compared to tissue. It follows that the disadvantageous tendency of low frequency systems to produce unwanted arcing when the electrode is outside tissue is avoided.
By tailoring the length of the active electrode according to the delivered frequency and the expected relative dielectric constant of the tissue to be treated, the voltage step-up effect can be maximised. Specifically, if the electrode represents a quarter wavelength in tissue (measured, for instance, from the end of a coaxial feed line to the tip of the electrode), an approximate voltage minimum at the proximal end of the electrode is transformed into a voltage maximum at the tip. For an operating frequency of 2.45 GHz, then, the active electrode length is preferably in the region of 4 mm. A diameter of less than 1 mm, and advantageously less than 0.5 mm, is preferred. It will be appreciated that the most advantageous length of the electrode depends not only on the operating frequency and the dielectric constant of the tissue being treated, but also on the surgical technique. i.e. the depth to which the electrode is immersed in tissue. As surrounding tissue is vaporised, the effective dielectric constant reduces, causing the electrode voltage to fall, thus reducing the applied voltage, contrary (as described above) to the conditions tending to promote arcing. An electrode with such dimensions tends, therefore, to be self-regulating, only delivering high voltages when applied to the tissue.
From a method aspect, the invention provides a method of electrosurgically cutting tissue comprising applying to the tissue an electrode assembly having an exposed active electrode electrically coupled to a radio frequency (r.f.) source, activating the source to feed UHF energy to the electrode at a voltage level sufficient to cause cell rupture within the tissue due to dielectric heating of the tissue without arcing at the exposed electrode. As explained above, a capacitive return path is preferably provided for the UHF current by providing a return electrode adjacent the active electrode and arranging for the return electrode to be located adjacent but spaced from the tissue during the procedure. The return electrode may be coupled directly or capacitively to the r.f. source (e.g. by direct or capacitive connection to the outer supply conductor of a coaxial feed structure).
Improved electric field strength may be achieved by inserting a major part of the projecting electrode portion into the tissue to be treated, typically to a depth of at least xcexs/8 where xcexs is the wavelength of the supplied UHF energy in the tissue.
The invention also includes an electrosurgical instrument for cutting tissue at a UHF operating frequency, wherein the instrument comprises a feed structure and an electrode assembly coupled to receive radio frequency (r.f.) power from the feed structure, wherein: the feed structure comprises a coaxial line having an inner supply conductor and an outer supply conductor, the electrode assembly comprises an active electrode and a return electrode, the active electrode comprises an exposed elongate electrically conductive member which is coupled to the said inner conductor and is less than xcex/8 in length, where xcex is the wavelength in air corresponding to the operating frequency, and the return electrode is coupled to the outer supply conductor and located so as to be adjacent to active electrode and set back from the distal end of the active electrode, whereby the feed structure and electrode assembly together constitute an open-circuit coaxial line, and means isolating the electrode assembly from at least a part of the outer supply conductor in respect of unbalanced currents at said operating frequency. Such an instrument may comprise a handpiece, an elongate instrument, a shaft comprising an rigid or resilient coaxial feeder forming at least part of the coaxial line, the shaft having a proximal end secured in the handpiece and a distal end mounting the electrode assembly. The active electrode member, typically in the form of a rod, may extend directly from the end of the coaxial line.